Configuration of gate to drain (GD) clamp and ESD protection circuit for power device breakdown protection

ABSTRACT

A semiconductor power device supported on a semiconductor substrate comprising a plurality of transistor cells each having a source and a drain with a gate to control an electric current transmitted between the source and the drain. The semiconductor further includes a gate-to-drain (GD) clamp termination connected in series between the gate and the drain further includes a plurality of back-to-back polysilicon diodes connected in series to a silicon diode includes parallel doped columns in the semiconductor substrate wherein the parallel doped columns having a predefined gap. The doped columns further include a U-shaped bend column connect together the ends of parallel doped columns with a deep doped well disposed below and engulfing the U-shaped bend.

This patent application is a Divisional Application and claims thePriority Date of a co-pending application Ser. No. 12/378,039 filed onFeb. 9, 2009 by common Inventors of this application. The Disclosuresmade in the patent application Ser. No. 12/378,039 are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the layout and manufacturing processof the semiconductor power devices. More particularly, this inventionrelates to an improved configuration of gate-to-drain (GD) clampcombined with the electrostatic discharge (ESD) protection circuit forpower device breakdown protection to achieve smaller die size, reducedleakage currents, better control of the GD clamp breakdown voltage andlower production cost.

2. Description of the Prior Art

Conventional layout for manufacturing semiconductor power devices withprotection circuits against breakdown and also against electrostaticdischarges (ESD) still has a limitation. A general practice is to placethe multiple gate-to-drain Zener diodes at the periphery of the die.These gate-to-drain Zener diodes may be made in the same way asgate-to-source ESD diodes. The configuration causes the die size toincrease and therefore increases the production costs of the powerdevices. Another technical difficulty is related to the great width ofthe gate to drain Zener diode. The leakage current Idss is proportionalto the width of the Zener diodes in this configuration. The great widthof the Zener diodes thus makes it difficult to limit the Idss leakagecurrent under 10 μA that is generally required in many of such powerdevice applications.

FIG. 1 is a top view of the gate-drain clamp for the device as generallyimplemented in the conventional semiconductor power device. The gate todrain clamping voltage is implemented with a plurality of Zener diodes110 on the periphery of the device. The multiple Zener diodes as shownthus occupy large areas of the die. Furthermore, as discussed above, theZener diodes formed around all the peripheral edges have a very largewidth thus causing the drain-source leakage Idss current to increasesignificantly and adversely affect the performance rating of theclamping circuits.

The function of a gate-to-drain (GD) clamp is to work with a gateresistor to turn on the FET before the drain to source voltage reachesavalanche breakdown that can potentially cause permanent damage to FET.The GD clamp function as disclosed by U.S. Pat. No. 5,365,099 isimplemented with back-to-back polysilicon diodes alone. The back-to-backpolysilicon diodes are usually made as alternating P and N stripes.However, such device has a drawback that the polysilicon diodes occupytoo much space wherein each stripe has a width of approximately fivemicrons for protecting a breakdown voltage up to six volts.

Shen et al. disclose in U.S. Pat. No. 5,536,958 a semiconductor devicethat has an improved high voltage protection by including an integratedSchottky diode in conjunction with a plurality of back-to-backpolysilicon diodes to limit the voltage potential that may arise betweenthe gate and the drain terminal. In another embodiment of U.S. Pat. No.5,536,958, a contact region is formed in the substrate rather than aSchottky diode to contact the back-to-back diodes to support some of thevoltage with a pinch-off effect with the remainder of the voltagesupported by the substrate. The structure is able to support excessivevoltage in the conduction mode rather than the avalanche mode.Furthermore, in the 5^(th) International Symposium of PowerSemiconductor Device and IC in May, 1993. Yamazaki et al. disclose anover voltage protection circuit by integrating a silicon avalanche diodewith an insulated-gate bipolar transistor (IGBT) structure that furtherincludes a polysilicon Zener diode. With polysilicon diodes combinedwith Schottky diode or silicon diodes in these disclosures however thedevices are limited to high breakdown voltage applications. Furthermore,the breakdown voltage for these types of clamping devices is difficultto control. Therefore, the GD clamping functions for device with low,well-controlled breakdown voltage applications with silicon diodes tosupplement the back-to-back polysilicon diodes with space efficientconfigurations are still not available. Another problem with the priorart methods of GD clamping is that the breakdown voltage at the ends ofthe silicon diodes have lower breakdown voltage (BV) than the rest ofthe silicon diode regions. This will adversely affect the performance ofthe GD clamping by allowing current flow before the desired BV. Thelowered BV is due to the higher electric fields that are present at theends of P-N junctions.

A GD clamp comprising of polysilicon diodes alone occupies too muchspace. A conventional GD clamp comprising polysilicon diodessupplemented with a silicon or Schottky diode as disclosed in the priorart has difficulties in controlling the breakdown voltage. Therefore, itis necessary to provide an improved layout for the ESD and thegate-drain clamping circuits on the semiconductor power device such thatthe above discussed difficulties and limitations can be overcome.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide a new andimproved layout of an ESD protection circuit combined with gate-drainclamping on a semiconductor power device by forming a combination ofgate to drain Zener diodes, a silicon diode and a small gate resistorfor power MOSFETs' breakdown protection. The gate to drain Zener diodeis only located at the one side of the die. Therefore, die size isreduced as compared to conventional design. Since the width of the gateto drain Zener diode is reduced, Idss leakage is low. No additionalmasks are needed as compared to a conventional power MOSFETs withpolysilicon ESD diodes process. Gate to source Zener diodes may be addedfor ESD protection without any additional cost. It is another aspect ofthis invention that it provides a method to achieve a low Idss leakage,low resistance in gate to drain clamp with a smaller die size,therefore, low cost. It is another aspect of this invention to achieve awell-controlled breakdown voltage for the GD clamp, which is scalable tolow voltage applications.

Briefly in a preferred embodiment this invention discloses asemiconductor power device supported on a semiconductor substratecomprising a plurality of transistor cells each having a source and adrain with a gate to control an electric current transmitted between thesource and the drain. The semiconductor further includes a GD clampcircuit connected in series between the gate and the drain furtherincludes a plurality of back-to-back polysilicon diodes connected inseries to a silicon diode which includes parallel doped columns in thesemiconductor substrate wherein the parallel doped columns having apredefined gap. In an exemplary embodiment, the doped columns having agap ranging between 2 to 5-micrometers between the doped columns forachieving a breakdown voltage ranging from 63 to 75 volts. In anotherexemplary embodiment, the doped columns further include a U-shaped (asseen from above) bend connecting the ends of the parallel doped columnstogether. In an alternate embodiment a doped well of the sameconductivity type as the parallel doped columns is disposed below andabout the ends of the columns and engulfs the U-shaped bend in an endwell, which quells the tendency for the ends of the doped columns tohave a lower breakdown voltage than the rest of the columns. In anotherembodiment the back-to-back polysilicon diodes are connected on one endin series to the silicon diode by a floating metal, and on the other endto a gate metal; this gate metal connects to the gate electrode (gatepad) through a gate resistor Rg, and directly contacts gate runnertrenches. In another exemplary embodiment, the GD clamp terminationdisposed on only one side near an edge of the semiconductor substratesupporting the semiconductor power device. In another exemplaryembodiment, the GD clamp termination is disposed on only one side nearan edge of the semiconductor substrate for disposing a gate padconnected to the gate via a plurality gate runners extended thereto. Inanother exemplary embodiment, the semiconductor substrate furtherincludes a deep doped well disposed below the silicon diode. In anotherexemplary embodiment, the semiconductor substrate further includescontact openings opened above the doped columns of the silicon diode andfilled with a contact metal for connecting to the silicon diodes formedas doped columns in the semiconductor substrate. In another exemplaryembodiment, wherein the contact metal for connecting to the silicondiodes is a floating metal connected between the back-to-backpolysilicon diodes and the doped columns of the silicon diode. Inanother exemplary embodiment, the semiconductor substrate furtherincludes a floating well (optionally deep wells) disposed next to achannel stop near an edge of the semiconductor substrate. In anotherexemplary embodiment, the back-to-back diodes includes a plurality ofalternating doped regions in a polysilicon layer disposed on top of aninsulation layer above the semiconductor substrate. In another exemplaryembodiment, the semiconductor power device further includes agate-to-source (GS) ESD protection circuit comprising gate-to-source(GS) back-to-back Zener diodes. In another exemplary embodiment, the GDclamping termination and GS ESD protection circuit are processed with noadditional manufacturing masks. The GD clamping Zener diodes are formedusing the same masks as the GS ESD structure.

This invention further discloses a method for clamping a gate-to-drainvoltage of a semiconductor power device supported on a semiconductorsubstrate comprising a plurality of transistor cells each having asource encompassed in a body region and a drain with a gate to controlan electric current transmitted between the source and the drain. Themethod further comprises a step of interconnecting a gate-to-drain (GD)clamp circuit between the gate and the drain by forming a plurality ofback-to-back diodes connected in series to a silicon diode comprisingdoped regions in the semiconductor substrate and connecting theback-to-back diodes to the gate. In an exemplary embodiment, the methodfurther includes a step of forming the doped regions as doped columns inthe semiconductor substrate to function as the silicon diode connectingto the back-to-back diodes. In another exemplary embodiment, the methodfurther includes a step of forming the doped regions as doped columnsconnected on one end of the columns by an U-shaped bend doped-region tofunction as the silicon diode connecting to the back-to-back diodes

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top schematic view of a corner of a conventional device toshow the structural features of a conventional GD clamp.

FIG. 2 is a circuit diagram of an improved GD clamp circuit of thisinvention.

FIG. 3A is a top view of a MOSFET device with an improved GD clamp,FIGS. 3B-1 and 3B-2 are explosive views of the lower-left corner of thedevice in FIG. 3A, and FIGS. 3C and 3D are cross sectional views takenalong cross sections A-A and B-B, respectively, of FIG. 3B-2.

FIG. 4 is a cross sectional view of an alternative embodiment of theinvention, taken along cross section A-A of FIG. 3B-2.

FIG. 4A is a top view of an alternative embodiment of the invention.

FIG. 4B is a cross sectional view of an alternative embodiment of theinvention taken along cross section C-C of FIG. 4A.

FIG. 5 is a top view of a MOSFET device with an improved GD clamp, alongwith a gate-to-source ESD protection circuit.

FIG. 6 is a top view of a MOSFET device with an improved GD clamp, alongwith a gate-to-source ESD protection circuit, with an alternate layoutof the gate metal.

FIGS. 7A-7I are a series of cross sectional views showing a method ofmanufacturing a MOSFET device with an improved GD clamp.

DETAILED DESCRIPTION OF THE METHOD

FIG. 2 is a circuit diagram to illustrate the breakdown protectioncircuit of a MOSFET device 100 of this invention. The MOSFET device hasa gate electrode 101, a source electrode 102 and drain electrode 103.The GD clamp circuit applies a combination of gate to drain Zener diodepairs, shown as polysilicon diodes 110 with a parasitic resistance Rz2115, a silicon diode 120 and a small gate resistor Rg 125 to serve thefunction of breakdown protection for the power MOSFET. The Zener diodepairs 110 comprise back-to-back Zener diodes. In addition to the GDclamp circuit, the breakdown protection circuit may further comprise agate-to-source ESD protection circuit including gate-to-source Zenerdiode pairs 130 with parasitic resistance Rz1 115. FIGS. 3A-D belowfurther illustrate that the gate to drain Zener diode pairs 110 that areformed at only one side of the die. Therefore, die size is reduced ascompared to conventional design. The GD clamp circuit is electricallyconnected to the drain electrode 103 disposed on the bottom surface ofthe substrate wherein the electrically connections are well know and notspecifically shown herein the drawings. Since the width of the gate todrain Zener diodes 110 is reduced, Idss leakage is low. The gate tosource Zener diode pairs 130 may be added for ESD protection without anyadditional cost. No additional masking steps are required in themanufacturing processes of forming the GD clamp compared to conventionalmanufacturing process for providing ESD protection for the powerMOSFETs.

FIG. 3A is a top view of a semiconductor die with improved GD clampcircuit of this invention. FIGS. 3B-1 and 3B-2 are explosive views ofthe bottom-left corner of FIG. 3A, and FIGS. 3C and 3D are crosssectional views of cross sections A-A and B-B, respectively, of FIG.3B-2. Oxide and passivation layers are not shown in FIGS. 3A, 3B-1 and3B-2 to avoid obscuring the details. FIG. 3B-2 is a top view of the samearea as FIG. 3B-1, except that the metal layers are made transparent inFIG. 3B-2 to better illustrate the polysilicon diodes 110 and thesilicon diode 120. The outlines of the polysilicon diodes 110 and thesilicon diode 120 are shown in FIG. 3B-1. The GD clamp circuit is formedon only one side of the die to reduce the area it occupies on the die.This allows more of the die area to be used as active area. The gateelectrode 101, e.g. the gate pad, is connected to gate metal 111 throughthe gate resistor Rg 125. The gate metal 111 encircles the source metal102, and is directly connected to gate runner trenches 131. The gatemetal 111 and the source metal 102 are separated by a metal gap. Thegate metal 111 is connected to one side of the gate-drain polysilicondiodes 110. The polysilicon diodes 110 are connected on the other sideto the silicon diode 120 through a floating metal 118. The floatingmetal 118 connects the silicon diode 120 in series with the polysilicondiodes 110. As shown in FIG. 3C, the silicon diode is a PN junction withthe silicon diode implant having the same conductivity type as the bodyregions, P-type for n-channel FET. In this case, the N side of the diodeis the epitaxial layer 145, which sits on the substrate 150 functioningas the drain for the MOSFET device. As an aside, the epitaxial layer 145and the substrate 150 are sometimes referred to together as thesemiconductor substrate. A channel stop region 160 may be formed at theedge of the semiconductor die.

The silicon diodes 120 are formed as implant regions in the substratethrough contact openings 121 in an oxide layer. Floating wells 140provide isolation. The floating wells 140 may form a ring surroundingthe silicon diode 120. The implant region to form the silicon diode 120and the floating wells 140 are of the same conductivity type as the bodyregion of the MOSFET. The channel stop 160, floating wells 140, silicondiode 120, and gate runner trenches 131 are all formed in the epitaxiallayer 145 of the die. The epitaxial layer 145 is formed over thesubstrate 150. The drawings presented here are not to scale. Theepitaxial layer 145 and the substrate 150 are of the same conductivitytype as the source and serve as the drain of the MOSFET. A drainelectrode 103 is connected to the bottom of the substrate 150. Thesilicon diode 120 comprises two parallel doped columns, separated by apre-set gap, d. In a preferred embodiment, the parallel doped columnsfurther comprise a U-shaped bend 122 (FIGS. 3B-1 and 3B-2) to connectthe ends of the columns together. Sharp angles and corners bring abouthigher electric fields, which lower the breakdown voltage. Thus the endsof regions tend to have lower breakdown voltages than the rest of theregion which can cause undesired effects such as current leakage andpremature turn-on. The U-shaped bend 122 helps to alleviate this effectand allows for a more controlled and uniform breakdown voltage. In anexemplary embodiment as shown, the ends of the parallel doped columns ofthe silicon diode 120 are further encompassed by an end well 140-1, asshown in FIG. 3D. The end well 140-1 has the same conductivity type asthe parallel stripes of the silicon diode 120, but a lower dopingconcentration, and so further improves control of the breakdown voltageby raising the BV at the ends of the silicon diode 120 to above thedesired BV. This way the ends of the silicon diodes will not cause theabove discussed problems of leakage and pre-mature turn-on. The U-shapedbend 122 and the end well 140-1 provide better control to the overallbreakdown voltage. The spacing of the silicon diodes allows one toadjust the breakdown voltage (BV). A bigger gap reduces the BV andvice-versa because of the changes of the electrical field. For example,a gap of 2 micrometers has an overall BV of 75 V and a gap of 5micrometers will have an overall (Zener+silicon) BV of 63.7 V. For theBV of the silicon diode alone, a 2 micrometers gap may have a BV of 44.5V, and a 4 micrometers gap may have a BV of 35.6 V

FIG. 4 shows a cross section of an alternative embodiment of thisinvention. Like FIG. 3D it is taken along cross section A-A of FIG.3B-2. In this embodiment, a shallow silicon etch is performed throughthe contact openings 120-1 and 120-2 prior to implanting the silicondiode 120. This etching process creates a configuration that includesextra corners and edges into the parallel doped columns which increasesthe electric field and lowers the breakdown voltage of the silicon diode120. The contact region (anode type region of silicon diode) depth isvery shallow, about 0.1-0.2 micrometers after implant and diffusion. Asdescribed above, a shallow contact region of the diode has lower BV.

FIG. 4A shows a top view of an alternative embodiment of this invention,and FIG. 4B shows a cross sectional view taken along cross section C-Cof FIG. 4A. They are similar to FIGS. 3B-2 and 3D, but its silicon diode120′ only comprises a single column rather than two parallel columns.The column is implanted through contact opening 121′ in the oxide. Theend of the column for silicon diode 120′ is enclosed in an end well140-1 that serves to improve control of the breakdown voltage of thesilicon diode 120′. To those skilled in the art, it is apparent that theconcept of enclosing the ends of a diode in a deep well can improvecontrol of the breakdown voltage regardless of the design of the diode.For example, the diode may comprise one, two, or more columns.Alternatively, the ends of the columns may not be connected in aU-shaped bend, but are still enclosed within the end well 140-1. Thefloating wells 140 and end well may be formed as deep well with a depthof about 2 micrometers. The floating wells and end well may notnecessarily be deep well and can also be formed with the same depth asregular body region. By way of example, a device with a single deep wellguard ring with a spacing of 4 micrometers from the silicon diode 120may have a BV of 97V.

FIG. 5 shows a top schematic view of an alternative embodiment of theinvention. The semiconductor die has a GD clamp circuit, and also agate-to-source (GS) ESD protection circuit 130. The GS ESD protectioncircuit 130 is well known in the art. To one skilled in the art, it isapparent that the GS protection circuit 130 may be formed using the samemanufacturing steps as the GD clamp, so there is no extra cost. FIG. 6shows the top view of another alternative embodiment of this invention.It is similar to FIG. 5, but the gate metal 111′, includes an extrasection 111-1′ that helps channel gate-to-drain current towards the gateresistor Rg 125.

FIGS. 7A to 7I are a series of cross sectional views showing a methodfor forming the improved GD clamp. FIG. 7A starts with an epitaxiallayer 745 over a substrate 750. Gate trenches (not shown) and gaterunners 731 are formed in the epitaxial layer 745. Then a polysiliconlayer 730 is grown over an oxide layer 715 and patterned as shown FIG.7B. The polysilicon layer 730 is doped the same conductivity type as thebody region of the MOSFET. A body mask is applied and a body implant 723is carried out to form the body regions 722 and floating well regions740 and end well 740-1 as shown in FIG. 7C-D. In FIG. 7D the implantsare diffused out to form the floating well regions 740, end well 740-1and body regions 722. If the floating wells 740, and end well 740-1 aredeep wells, then an extra mask and diffusion may be needed to form them.Optionally the body regions 722, floating well regions 740 and end well740-1 may be formed before the polysilicon layer 730 and oxide layer 715are formed.

In FIG. 7E a source mask is used to implant and form source regions (notshown), stripes 724 in the polysilicon layer 730, and channel stop 760.The polysilicon layer 730 now has alternating P and N type stripes, thusforming the back to back Zener diodes 710 of the GD clamp. An insulatinglayer 725, such as an oxide, is deposited and patterned to form contactopenings 732 as shown in FIG. 7F. In FIG. 7G a body contact implant iscarried out through contact openings 721 to form body contacts (notshown) and the silicon diode 720, followed by metal deposition andpatterning in FIG. 7H to form the top metal layers: source metal (notshown), gate metal 721, and floating metal 718. In FIG. 7I the backsidemetal is deposited, forming the drain metal 703. It should be apparentof one skilled in the art that forming the improved GD clamp requires noextra manufacturing steps compared with forming a standard GS ESDprotection circuit having back-to-back polysilicon diodes. Therefore theGD clamp may be formed on a MOSFET device having a GS ESD protectioncircuit for no extra manufacturing cost.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. For example, other conductivematerial instead of polysilicon may be used. The technique can apply toboth N- and P-channel MOSFETs and LDD MOSFETs. Accordingly, it isintended that the appended claims be interpreted as covering allalterations and modifications as fall within the true spirit and scopeof the invention.

1. A method for clamping a gate-to-drain voltage of a semiconductorpower device supported on a semiconductor substrate comprising aplurality of transistor cells each having a source encompassed in a bodyregion and a drain with a gate to control an electric currenttransmitted between the source and the drain, wherein the method furthercomprising: interconnecting a gate-to-drain (GD) clamp circuit betweensaid gate and said drain by forming a plurality of back-to-backpolysilicon diodes connected on a first side in series to a silicondiode comprising at least one doped column in said semiconductorsubstrate and connecting said back-to-back polysilicon diodes on asecond side to a gate electrode through a gate resistor.
 2. The methodof claim 1 further comprising a step of: forming parallel doped columnsin said semiconductor substrate to function as said silicon diode,wherein the parallel doped columns having a predefined gap forcontrolling breakdown voltage.
 3. The method of claim 2 wherein: saidstep of forming said parallel doped columns further comprises connectingthe ends of the doped columns together with a U-shaped bend.
 4. Themethod of claim 1 further comprising a step of: forming an end wellunder the ends of said at least one doped column, wherein the end wellhas the same conductivity type as the doped column and has a lowerdoping concentration than the doped column.
 5. The method of claim 1wherein: said method requires no extra manufacturing steps compared tomanufacturing a semiconductor power device with a standard GS ESDprotection circuit having back-to-back polysilicon diodes.